1. Field of the Invention
This invention relates to an image processing apparatus for processing a n-valued image i.e., an image represented by data having n bits per pixel), such as a binary or ternary image data, by further converting this image data into multivalued image data.
2. Description of the Prior Art
In a case where an image is processed by this kind of apparatus after being converted into a binary image (i.e., one represented by binary data), there is a need for converting the binary image into a multivalued image.
Specifically, for color processing such as masking on a binary image, it is necessary to simultaneously calculate three color images by using three color components, and a technique for converting the binary image into a multivalued image is indispensable.
As a conventional technique for converting a binary image into a multivalued image, a method has been proposed in which the number of dots formed within a rectangular region having a certain size is counted, to obtain the proportion of the region occupied by the dots (hereinafter referred to as "area rate"), and the density is determined by the area rate.
For example, if 8 dots are formed in 4.times.4 pixel region and if the density range is 8-bit, that is, there are 256 gradations, a density of EQU 256.times.8/(4.times.4)=128
is obtained. In this example, the area rate is "8/(4.times.4)".
In this conventional method, however, an average density is calculated for each region. That is, this method has the same effect as filtering the image using a smoothing filter. Edge portions and the entire resulting image are therefore blurred.
In particular, in a case where the input binary image corresponds to characters or pictures, the resolution is reduced and the quality of the resulting image is considerably deteriorated.